Group A streptococci are significant human pathogens associated with a variety of diseases including pharyngitis, impetigo, and invasive infections such as cellulitis and bacteremia. Recently, the incidence and severity of streptococcal disease has increased and the exact nature of the invasive phenotype has not been fully elucidated. Studies in Dr. Boyle's laboratory have shown that group A streptococci can interact with the human plasmin(ogen) system by a number of different mechanisms to acquire surface proteolytic enzymatic activity, which cannot be inhibited by physiological protease inhibitors like alpha-2 antiplasmin. These pathways include direct binding of plasmin to surface binding proteins, as well as a more complicated pathway involving two host proteins (plasminogen and fibrinogen) and two bacterial proteins (the secreted bacterial plasminogen activator, streptokinase (SK), and a surface fibrinogen-binding protein). Preliminary evidence, using a mouse model of skin infection, suggests that acquisition of this host protease activity enhances the ability of group A streptococci to cause a systemic infection. The significance of the interaction of group A streptococci with the human plasmin(ogen) system will be studied by: 1) establishing the host and bacterial factors required for acquiring surface plasminogen activator and plasmin(ogen)-dependent enzymatic activity under conditions that would exist in the infected host; and 2) determining the consequences of the acquired enzymatic activity in a mouse model of skin infection. These studies will use an established mouse model of skin infection using outbred CD1 mice as well as studies in transgenic plasminogen gene knock-out mice reconstituted with mouse or human plasminogen. Regulation of key bacterial factors involved in acquisition of host plasmin(ogen)-dependent activity under conditions that mimic those group A streptococci could encounter in the infected host will also be analyzed. These studies will lead to a clearer understanding of the interaction of group A streptococci with the human plasmin(ogen) system and the importance of this interaction on the course, severity, and nature of invasive infections. With the reported increase in mortality and morbidity associated with invasive streptococcal infections, a more comprehensive understanding of the pathogenic process is important.